Our overall objective is to identify and understand the factors that control the balance of energy derived from endogenous carbohydrate, lipid, and amino acid sources during sustained, submaximal exercise. A related objective is to understand how various life situations (physical fitness, diet, gender, age) affect the balance of substrate utilization (partitioning) in humans. The theoretical basis of our approach is the 'Crossover Concept' which postulates that during rest and mild to moderate intensity exercise in the post-absorptive state, lipids provide the greatest proportion of energy for muscle and the body at large. However, as the exercise intensity increases from moderate to hard to maximal, the balance of substrate utilization in working muscle switches, or 'crosses over' from lipid to carbohydrate. Using this concept, we seek to describe the mechanisms by which exercise, exercise training, ovarian steroids, age, and dietary history affect the balance of substrate utilization. In pursuit of our overall objective, we propose to explore two specific aims. These are to: (1) describe the interactive effects of exercise intensity and endurance training on muscle and whole body fatty acid oxidation; and (2) evaluate the effects of aging and ovarian sex steroids on the balance of substrate utilization. With these data we shall be able to expand our model of substrate utilization. To assess effects of exercise intensity and to make comparisons at given relative and absolute exercise intensities before and after endurance training, for Aim 1 young men will be studied before training at power outputs (P0) that elicit 45 and 65% VO2peak; after training they will be studied at the P0 that elicited 65%V02peak before training, and at the new 65%V02 peak. To assess acute and long-term metabolic and enzymatic responses at whole-body and working muscle (leg) levels we will use the combination of tracers {[1-13C]palmitate, [1,1,2,3,3-2H2]-glycerol (D5-glycerol), and D2-glucose}, indirect calorimetry, (a-v) measurements, biopsies, and, possibly, NMR spectroscopy. For Aim 2 studies on older men, young amenorrhic as well as older postmenopausal women we will use tracers {[1-13C]palmitate, D5-glycerol, D2- and [1-13C]glucose, as well as [1-13C]leucine}, and indirect calorimetry. On older women longitudinal designs will be employed to assess effects of training and HRT on substrate partitioning. Age-matched males will also be studied. All techniques, whether they involve exercise physiology, indirect calorimetry, tracer infusion and blood sampling, metabolite derivatization, isotopic enrichment determination by GC /MS, metabolite and hormone assays, Western blotting and dietary control, are highly developed by the investigative team. Further, we have the auxiliary personnel and facilities to conduct longitudinal training studies